Well, if we look at the book Yali's question is actually restated in a number of different ways as the book progresses, each of which of course allows Diamond to cast new light on world history and the way that certain conditions favoured certain peoples over others. However, perhaps the most encompassing rewording comes in the Prologue, which can be found in the following paragraph:
Thus, questions about inequality in the modern world can be reformulated as follows. Why did wealth and power become distributed as they now are, rather than in some otehr way? For instance, why weren't Native Americans, Africans, and Aboriginal Australians the ones who decimated, subjugated, or exterminated Europeans and Asians?
This is, essentially, Yali's question in a nutshell, and the rest of the excellent book is Diamond's attempt (and a very convincing one at that) to answer it. Yali's question essentially concerns the reason behind inequalities in history, and his rephrasing of Yali's question in the above quote helpfully gives Diamond a good frame of reference with which to approach this massive issue.